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Dear Tom, Could you explain what causes some clouds...

Could you explain what causes some clouds to be snow white. Also, can white clouds ever be the source of rain?

Kenneth Conway, Chicago

Dear Kenneth,

The color of clouds depends mainly on the viewers perspective relative to the source of the light which is illuminating the clouds. During the day, sunlight is of course the only source of light. Much of the sunlight which strikes a clouds surface reflects off of it, and when we see that reflected light the clouds surface appears to our eyes to be a brilliant white.

All this happens independently of weather conditions within or underneath the cloud. If we are seeing the brilliant white light reflected from a cumulonimbus cloud, there may be a raging thunderstorm in progress beneath it. If we are looking at small, puffy cumulus clouds also brilliant white then the weather at the moment is usually pleasant and dry.

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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.